Thursday, April 12, 2012

Reservoir Rock In World's Biggest Oil Field Is Made Of Shit

Most massive and nonporous limestones contain textures made by
invertebrate animals that ingest sediment and turn out fecal pellets.
Usually, the pellets get squished into the mud. Rarely do the fecal
pellets themselves form a porous sedimentary rock. In the 1970s, the
first native-born Saudi to earn a doctorate in petroleum geology arrived
for a year of work at Princeton. I used the occasion to twist Aramco’s
collective arm for samples from the super-giant Ghawar field. As soon as
the samples were ready, I made an appointment with our Saudi visitor to
examine together the samples using petrographic microscopes. That
morning, I was really excited. Examining the reservoir rock of the
world’s biggest oil field was for me a thrill bigger than climbing Mount
Everest. A small part of the reservoir was dolomite, but most of it
turned out to be a fecal-pellet limestone. I had to go home that evening
and explain to my family that the reservoir rock in the world’s biggest
oil field was made of shit.

A bewildering variety of particle types get bound together to form limestones. Post Cambrian times, the calcium carbonate shells of marine organisms have been the most common particle type, the primary building blocks of limestones. But other particle types like fecal pellets are also common.

For carbonate sedimentologists involved in oil exploration, understanding the origin and distribution of porosity and permeability i.e. the open spaces in which oil migrates and is naturally stored is the most important task. Sedimentologists recognize two broad categories of porosity. Primary porosity and secondary porosity. Primary porosity is the open space between the grains and forms during deposition of the sediment. Sediments that are deposited in environments where wave and tidal movements are vigorous will have high primary porosity because in such settings finer mud that can clog up interstices between coarser grains is winnowed away, leaving behind a lag of clean sand.

The image below is a photomicrograph of a fecal pellet sand from the Jurassic of England. The shining white material between the dark pellets is calcium carbonate cement which has filled up the primary porosity. Occasionally, there may be no precipitation of cement as the sand gets buried. In such situations the primary porosity is preserved and the deposit may become a reservoir rock.

Arrows and labels show
environments facing open ocean where currents and waves are vigorous and where primary porosity in sediment will be high. In the interior of the Bahamas, wave energy is much lower, resulting in sediment with less primary porosity. Fecal pellets may originate in the interior of platforms, in low energy settings. They often harden in these settings due to precipitation of cement in micro-pores within the grains. Often due to storms, these hardened pellets are then transported to high energy settings. Due to this early hardening, pellets resist getting squished against each other as the sediment is buried. Open spaces are thus preserved in such early hardened fecal pellet deposits.

Although the paleo-geographic setting would have been different than the Bahamas, the sediments of the Jurassic Ghawar reservoir limestone would have been deposited in high energy settings resulting in substantial primary porosity.

Another category of porosity is secondary porosity that forms due to the reaction of the sediment with water during burial. It results in open spaces being created by the dissolution and leaching away of mud and grains and also due to volume changes as calcite gets replaced by the denser dolomite. This type of porosity is also present in the Ghawar limestone.

Since 1950's the Ghawar field has produced over 65 billion barrels of oil. Daily production is about 5 million barrels, about 6% of global production.

One of the interesting things about Ghawar is the nature of its
reservoir which provides an argument against an ideology I fight all the
time, Young-earth Creationism. Ghawar is largely made of dung, which
would be hard pressed to be concentrated during a global flood and thus
contradicts the young-earth creationist claims.

A chaotic flood would have dispersed and broken up fecal pellets in to mud. Only long periods of wave action and winnowing and early cementation on a sea floor would have produce the well sorted fecal pellet sands of the Ghawar reservoir deposit.

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ABOUT THIS BLOG

I am a Sedimentary Geologist. On Rapid Uplift I write mostly about topics within the geosciences, but sometimes on biological evolution and environmental issues. I like to travel and in my free time I teach 12 year old kids soccer and rugby.